The M12 was originally meant to be an urban motorway serving the new town of Craigavon, starting on the M1 and burrowing into the town. It would have formed a C-shape as it passed through the urban area. When the M1 was built, the first section of M12 was built along with it, a short spur towards Craigavon to provide basic access to the motorway, and nothing was ever done to finish it. Indeed, the whole of Craigavon is basically half-built and has never reached anything like its intended population.
Because it was always meant to go much further the end of the M12 feels decidedly unfinished. Travelling out of Craigavon, motorway restrictions begin halfway underneath junction 2, where the road widens out to dual carriageway. Technically, non-motorway traffic can use the road until this point, from which there is no exit.
Junction 1 was originally envisaged to be a three-level interchange. In the 1960s it was opened with east-facing slips only, and a bridge was built over the M1 ready for its completion. In 1992, the junction was finished, but without actually bothering to build a third bridge to complete the interchange - instead using a flat roundabout to connect the three arms. It does look ridiculous, but it's functionally no different from a roundabout interchange - these kick off a number of mainland motorways like the M50 and M180.
Thanks to its mid-way roundabout, its strange narrowing to single-carriageway and its decidedly unfinished nature, the M12 might be one of the strangest and least motorway-like motorways anywhere in the UK.